This invention relates to document feeding apparatuses and, more particularly, it relates to an improvement in bottom level sheet feeding apparatuses.
Many modern, high speed copiers are capable of copying serially paginated booklets and the like, one page at a time, and presenting the copies as well as the copied pages in the proper sequential order. To accomplish this function, such devices commonly utilize bottom level document feeders which receive a stack of original documents to be copied, remove the lowermost sheet from the stack and convey it to the copying station. The advantage of a bottom level document feeder is that successive stacks of original sheets may be loaded into it, even as the sheets are being withdrawn from the lowermost stack, and the resultant copies and originals are presented in the same order in which they were loaded into the device.
There are many types of bottom level document feeders. For example, Strobel, U.S. Pat. No. 3,934,869, discloses a bottom level document feeder which includes a stack receiving tray, an endless belt conveyor partially extending into the tray, an abutment plate located downstream of the tray and above the belt, and a retard pad of the type disclosed in Stange, U.S. Pat. No. 3,768,803, located above the belt and downstream of the abutment plate. The abutment plate is oriented substantially perpendicularly to the portion of the belt conveyor which contacts and transports the sheets loaded onto the tray. The conveyor includes an idler roller located slightly upstream of the abutment plate and further upstream a belt roller which is located beneath the tray.
In operation, the conveyor urges a stack of sheets loaded onto the tray forwardly until their leading edges contact the vertical abutment plate. A gap beneath the plate allows the conveyor to displace a lowermost sheet or sheets forwardly beneath the abutment plate and into contact with the retard pad. The retard pad has a coefficient of friction sufficient to hold and thereby separate any additional sheets carried through the opening from the lowermost sheet and prevent their moving further with the lowermost sheet. Thus, only the lowermost sheet is conveyed past the retard pad by the belt, which has a coefficient of friction higher than that of the retard pad and thus can convey a sheet of paper past the retard pad.
Strobel also discloses the concept of connecting the upstream belt roller to a pivot arm for raising the belt. This feature is used to increase the surface area of the belt in contact with the bottom sheet of the stack for feeding sheets under abnormal conditions. Thus, a variable foot print feed belt is provided to accommodate varying sheet separating resistance.
Clausing in U.S. Pat. No. 4,174,102 also discloses a system for raising and lowering the upstream belt roller in a bottom level document feeder. In Clausing the upstream roller is an idler roll which is on a rotatable frame. When the lead edge of the bottom most sheet in a stack reaches the transport rolls, the frame carrying the idler roll is rotated so that the belt is removed from contact with the stack of sheets. When the trailing edge of the sheet being fed leaves the retard zone, the frame carrying the idler roll is rotated in the other direction so that the belt is again in contact with the bottom sheet of the stack. The apparatus is then ready to feed the next sheet and the lowering and raising cycle begins again. A baffle is used to maintain the proper entrance geometry when the belt is lowered away from the stack of sheets.
The stated purpose of raising and lowering the belt in the Clausing system is to prevent degradation of the retard pad, belt and sheet being fed. It is also stated that with the belt in its retracted position, away from the stack of sheets, the drive force on the next and subsequent sheets of the stack is very small and therefore the probability of multi-feed is reduced.
Thus, the systems of Strobel and Clausing are intended to overcome a problem inherent with many bottom level document feeding apparatuses; namely, that the conveying means for transporting the lowermost sheet from the stack within the paper tray often conveys more than a single sheet so that a number of sheets are conveyed past the retard station. This results in misfeeds and requires the sheet feeding operation to be stopped and the excess sheets removed from the apparatus downstream of the stack. An excessive number of such misfeeds results in significant downtime of the entire copying or duplicating machine, and possibly an unacceptable number of copy sheets which have either no text at all imprinted upon them, or text which is misregistered.
Another system for minimizing this problem is found in my copending application Ser. No. 529,455. As disclosed in that application, an idler roller is positioned to support the belt directly beneath a pinch point. The pinch point inhibits the passage of multiple sheets of paper and the system disclosed in my copending application, thus, reduces the overall number of misfeeds encountered.
It does not, however, totally eliminate them. Two problems remain. One is a problem termed reverse shingling. Reverse shingling occurs when the sheets in the stack, upon placement of the stack on the tray, slide relative to one another so that one of the upper sheets actually enters the pinch point first. Thus, the very bottom most sheet, which should be the first one to advance into the mechanism, is retained in the location of placement in the tray by the surface friction of the belt which extends partially into the tray. The upper sheets may then shift relative to that bottom sheets. This is true particularly when the stack of sheets is large and the surface friction between the sheets is low. This shifting may result in one (or more) of the upper sheets actually entering the pinch point first, i.e., out of order. If the sheet which does so is the second or third one in the stack, the corrective features provided by the retard pad should permit the bottom most sheet to catch up to and overtake the misfed lead sheet. However, if the sheet first entering the pinch point is actually the fifth or tenth one in the stack, the normal corrective features are not adequate. While an uncorrected misfeed caused by reverse shingling may occur only a small percentage of the time, it is still desirable to further minimize the problem.
The other possible problem is at the downstream end of the feed mechanism. It is desirable that the puller or transport rolls adjacent to the downstream end of the feed belt be independently driven. However, in that system if a misfeed occurs or if for some other reason the belt is to be stopped at a time when a sheet is in the nip of the puller or transport rolls, two situations can result, neither of which is desirable. If the drive for the belt is simply stopped, the puller or transport rolls which are independently driven will continue to pull the sheet therethrough and because the sheet couples to the belt by surface friction, the belt will continue to turn. On the other hand, if a brake is applied to the belt when the drive is stopped, the independently driven puller or transport rolls can drag the sheet over the surface of the non-moveable belt. This may cause irreparable damage to the sheet.
Accordingly, there is a need for further improvement in the sheet feeding apparatus of my copending application Ser. No. 529,455, so that an even more reliable bottom level document feeding apparatus is provided.